I use Photoshop, just because that is what I learned to use, long ago. I am frankly intimidated by the learning curve that GIMP seems to have. I also make extensive use of Fractal Terrains since nothing I draw can even come close to the generated landforms.

I am including Tablet and pen since I do most of my artistic endeavors on an old HP convertible tablet with the nifty Wacom-enabled tablet screen. I only rarely draw by hand though because the results are always depressingly craptacular.